1. Field of the Invention
The invention relates to gas separation using a pressure swing adsorption process. More particularly, it relates to the use of said process to enhance the recovery of two purified product fractions.
2. Description of the Prior Art
Pressure swing adsorption (PSA) processes and systems are well known in the art for achieving desirable separation and purification of a feed gas stream containing a more selectively adsorbable component and a less selectively adsorbable component. The more selectively adsorbable component is adsorbed as the feed gas is passed, at a higher adsorption pressure, over an adsorbent bed capable of selectively adsorbing said more selectively adsorbable component. Upon subsequent reduction of the bed pressure to a lower adsorption pressure level, the more selectively adsorbable component is desorbed from the adsorbent.
PSA processing is commonly carried out in systems containing more than one adsorbent bed, with each bed undergoing a processing sequence, on a cyclic basis, comprising (a) higher adsorption pressure feed - adsorption of the more selectively adsorbable component - discharge of less selectively adsorbable component, (b) lower pressure desorption and removal of the less selectively adsorbable component, typically from the feed end of the bed, and (c) repressurization of the bed to said higher adsorption pressure. PSA processing is particularly suited for air separation operations in a variety of industrial applications, particularly in relatively small sized operations for which the use of cryogenic air separation plants may not be economically feasible. PSA processing is also well suited to the drying of air or other gases.
In such drying applications, the moist gas is passed to the feed end of an adsorbent bed that preferentially adsorbs the water as the more selectively adsorbable component thereof. As water is removed from the adsorbent at the feed end of the bed, the bed becomes loaded with water and loses its adsorptive capacity with respect to additional quantities of moist gas passed therethrough. The following gas, depleted of moisture, encounters a zone of relatively dry adsorbent and emerges from the discharge end of the bed as a dry feed gas product. As such drying operations continue, an adsorption front of the more selectively adsorbed component, or mass transfer zone, moves through the bed from the feed end in the direction of the opposite, discharge end thereof, until the adsorption front reaches the vicinity of the discharge end and nearly all of the bed is water loaded. Before further drying can be achieved, the bed must be regenerated, i.e. the more selectively adsorbed water must be desorbed and removed from the bed. In PSA processing for drying, the selectively adsorbed water can be removed from the bed by depressurizing the bed from its higher adsorption pressure to a lower desorption pressure, typically by countercurrent depressurization in which gas is released from the feed end of the bed, and by flowing a dry purge gas through the bed from the discharge to the feed end thereof. The adsorption front of more selectively adsorbed water is thus driven back to the feed end of the bed. In a properly designed system, the concentration of the water impurity in the gas will increase when the pressure is reduced from its upper adsorption level, i.e. in a cocurrent depressurization step in which the pressure is decreased and gas is released from the discharge end of the bed. The amount of purge gas then required for desorption and removal of the more selectively adsorbed water is less than the amount of gas dried during adsorption. A portion of the dry air product is typically used as the purge gas, with the remaining dry air being removed from the system as the ultimate product stream.
Such conventional PSA processing is suitable for such air drying application because the moist feed air is freely available, and a high degree of recovery of the air is thus unnecessary. That is, the loss of air in the waste stream after purging is not of major importance. As those skilled in the art will appreciate, however, such a process may not be desirable or even satisfactory for the purification of a valuable gas that must be recovered without appreciable loss.
A typical PSA process for bulk gas separation is that for the production of oxygen from air by the selective adsorption of nitrogen and also of minor impurities such as water and carbon dioxide. The PSA processes used for such separation are conventional ones, similar to that referred to above for impurity removal, except that the more selectively adsorbed component, corresponding to the water impurity, is in very high concentration. This leads to short cycle times and a need for proper handling of the gases during the pressurization and depressurization steps. Representative examples of specific PSA processing cycles that have been used to produce oxygen from air are disclosed in the Batta patent, U.S. Pat. No. 3,717,974, and in the Hiscock et al. patent, U.S. Pat. No. 4,589,888. Again, such processes are satisfactory, at least in part, because the feed gas, i.e. air, is readily available, and a high degree of recovery of the product gas, i.e. oxygen, is not necessary for the economic feasibility of the air separation operation.
In the applications referred to above, i.e. air drying and air separation for the recovery of oxygen product, the more selectively adsorbable, or more strongly adsorbed, component is to be separated from the product gas constituting the less selectively adsorbable component, i.e. water from dry product air or nitrogen from oxygen product gas. This is typical of normal PSA processing. Such processing is not generally applicable for the purification of the more selectively adsorbable, or so-called heavy, component. Thus, the normal PSA processing cycles are satisfactory for the production of oxygen from air, but not for the production of nitrogen from air. While the gas released from the feed end of the bed is rich in nitrogen, compared to air, it is nevertheless too impure for most practical applications.
In other applications, it is desired to recover the more selectively, adsorbable or heavy component as the product gas, with the less selectively adsorbable or light component being removed as the impurity rather than as the desired product gas. The Wilson patent, U.S. Pat. No. 4,359,328, describes an inverted pressure swing adsorption process for this purpose. This process, desirably carried out in two or more beds, comprises a processing cycle of (1) low pressure adsorption, (2) pressurization to high pressure, (3) purging at said high pressure, and (4) depressurization for release of the more selectively adsorbed component as the desired product gas. In this process, the feed gas, e.g. air, is introduced into the adsorption bed at low pressure. The effluent from the bed, which is essentially the more selectively adsorbable component, is compressed to a high pressure, and a portion of this gas is used as a countercurrent purge gas to remove the less selectively adsorbable component from the bed. The remaining portion of said effluent gas is withdrawn as product gas, i.e. nitrogen in the case of air separation.
The process of the Wilson patent is superficially the inverse of the so-called normal PSA process. The inverted PSA process is different, however, in several important respects from the normal PSA process. After several cycles of operation of the inverted process, the heavy component will become concentrated in the low pressure effluent and on the product end of the beds. An adsorption-desorption front will be established in the bed, with the heavy component rich at the product end. The front can easily break through during the low-pressure portion of each cycle, thus allowing some of the light, i.e. less selectively adsorbable, component into the product stream. For this and other reasons, the inverted process needs to be operated for many cycles, possibly with a high reflux or purge ratio, before the heavy component becomes fully concentrated. Wilson discloses that, when the normal PSA process is optimized to enhance the nitrogen content of the low-pressure purge effluent, a nitrogen concentration of 88% was achieved, using air (80% nitrogen) as the feed gas. With the inverted process, Wilson discloses the concentration of the nitrogen to 96%, with a nitrogen recovery of 31.5%. This may be satisfactory for some applications in which the separation of air for the production of nitrogen is desired, even at relatively low nitrogen recovery. In other applications, however, the inverted PSA process of Wilson may not be able to concentrate and purify a valuable gas that must be obtained with a high level of product recovery.
Another means for concentrating the heavy, i.e. the more selectively adsorbable, component of a gas mixture is to employ a cocurrent-displacement type of PSA process. This type of PSA processing employs some of the features of both the normal and the inverted processes. Thus, the feed gas, e.g. air, is introduced into the feed end of the bed at high pressure and flows forward to the discharge end thereof, while the more selectively adsorbable, heavy component, i.e. nitrogen, is adsorbed on the bed. The light component, i.e. oxygen, of the gas stream passes through the bed and is discharged therefrom as a co-product or waste stream. The flow of feed gas, i.e. air, to the bed is stopped before the air-oxygen front, i.e., corresponding to the front of adsorbed nitrogen in the bed, reaches the discharge end of the bed. Nitrogen-rich product gas is then introduced into the feed end of the bed, which establishes a second front in the bed, i.e. a nitrogen-air front. This latter front moves faster than the air-oxygen front, which it eventually joins near the discharge end of the bed. At this point, the bed is loaded or saturated with the more selectively adsorbable nitrogen. Upon countercurrent depressurization from the feed end of the bed, this nitrogen is desorbed and is withdrawn from the feed end of the bed as the primary product. Further nitrogen product is obtained by purging the bed from the discharge end using some of the recovered oxygen as purge gas. The nitrogen thus produced is usually accumulated in a storage vessel, with some of said nitrogen gas being compressed and used as the cocurrent purge stream. Various other processing steps for pressure equalization and recycling are often employed to enhance the overall processing performance. Specific cocurrent displacement processes for the production of nitrogen from air have been disclosed in the Werner and Fay patent, U.S. Pat. No. 4,599,094, and in the Lagree and Leavitt patent, U.S. Pat. No. 4,810,265. These processes are capable of producing both nitrogen and oxygen from air with a high recovery of both components, with nitrogen usually being the principal product. In practical commercial applications using large diameter adsorbent beds, it is difficult to produce high-purity oxygen at the same time that high purity nitrogen is being produced.
While the co-current displacement cycles work well for obtaining nitrogen from air with a high product recovery, such cycles are not satisfactory for all circumstances encountered in the art. Thus, it is found that the process is not particularly effective when the heavy component is present in relatively low concentrations. In general, such co-current displacement processing can be carried out satisfactorily when the desorption of the more selectively adsorbable component is carried out under vacuum or at a pressure very much lower than the adsorption pressure, which pressure conditions may not be suitable or economical for many gas separation or purification applications.
Illustrative of other known PSA processes is the simulated moving bed process referred to in "Principles of Adsorption and Adsorption Processes" by D.M. Ruthven, Wiley and Sons, 1984, pp. 396-405. The process can produce high purity gases with high product recovery under some circumstances, but is relatively complex and costly, usually requiring the use of many adsorbent beds and valves. In addition, such processing often does not perform well when the adsorption isotherm of the heavy component is strongly curved or when there are several strongly adsorbed components with different equilibrium isotherms.
In another PSA processing approach described in the Keller and Kuo patent, U.S. Pat. No. 4,354,859, a feed gas is separated into two product streams by imposing cyclic pressure changes on both ends of an adsorbent bed. This process and system, referred to as a molecular gate, uses pistons to produce cyclic gas flows and pressure variations at the two ends of the bed, while the feed is admitted at an intermediate point. The volume displacements and phase angles of such opposing piston actions are adjusted to control the productivity and selectivity of the process. While the process and system enables two product streams to be produced, they are difficult to scale up to commercial size and to operate economically. As a result, the molecular gate approach has not been used for commercial gas separation or purification operations.
Thus, there remains a need in the art for improved PSA processing that is relatively simple, that uses only a few adsorbent beds and that can economically be employed to purify or separate a feed gas stream with high recovery of the desired product gas or gases.
It is an object of the invention to provide an improved PSA process for separating a multicomponent feed gas stream into two purified streams, without appreciable loss of the desired purified product gases.
It is another object of the invention to provide an improved PSA process for efficiently separating a binary gas stream into two pure gas fractions with a high recovery of both components of said gas stream.
It is another object of the invention to provide a PSA process for the removal of a more selectively adsorbable impurity from a less selectively adsorbable gas so as to produce a purified gas stream with a high recovery of the desired product gas.
It is a further object of the invention to provide a PSA process for the removal of a less selectively adsorbable impurity from a more selectively adsorbable gas so as to produce a purified gas stream with a high recovery of the desired product gas.
It is a further object of the invention to provide an improved PSA process for the removal of traces of nitrogen from an impure argon stream so that a purified argon stream can be produced with only a negligible loss of argon product in the purification process.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.